idée fixe

This French expression literally means "fixed idea", but can also be used in a stronger sense in the English language. It suggests an obsession or something that occupies someone's mind for a long time. Sometimes used to describe a theme that an artist, writer or composer returns to periodically within a work or during a career. Introduced in Oxford English Dictionary in 1836

Example: "Man, this French stuff has really become an idée fixe for this bigmouth guy."

Idéfix is also the name of Obelix' dog in the French cartoon Asterix. I have no idea if has anything to do with idée fixe.
This is an entry in the mini series about French idioms in the English language.

2002.04.26@12:59 thbz says re idée fixe: The name Idéfix does correspond to "idée fixe". All the character names in Astérix are puns, but there is usually no relation between the meaning of the word and the character.
2003.3.14@10:58 Siobhan says re idée fixe : re Idéfix, the little dog: i can't give you sources and it might be an urban legend, but it sounds plausible enough. they say that one of the two drawers of Asterix et Obelix always drew this little dog, in the most inappropriate places. it was his idee fix. in the end they warmed to him and kept him as obelix' dog. from doodle to dog so to speak :)